Power semiconductor devices and methods for manufacturing the same

ABSTRACT

A power semiconductor device that realizes high-speed turnoff and soft switching at the same time has an n-type main semiconductor layer that includes lightly doped n-type semiconductor layers and extremely lightly doped n-type semiconductor layers arranged alternately and repeatedly between a p-type channel layer and an n + -type field stop layer, in a direction parallel to the first major surface of the n-type main semiconductor layer. A substrate used for manufacturing the semiconductor device is fabricated by forming trenches in an n-type main semiconductor layer 1 and performing ion implantation and subsequent heat treatment to form an n + -type field stop layer in the bottom of the trenches. The trenches are then filled with a semiconductor doped more lightly than the n-type main semiconductor layer for forming extremely lightly doped n-type semiconductor layers. The manufacturing method is applicable with variations to various power semiconductor devices such as IGBT&#39;s, MOSFET&#39;s and PIN diodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/539,295,filed on Aug. 11, 2009. Furthermore, this application claims the benefitof priority of Japanese application 2008-209000, filed Aug. 14, 2008.The disclosures of these prior U.S. and Japanese applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices and methods formanufacturing the semiconductor devices. Specifically, the inventionrelates to semiconductor devices such as insulated gate bipolartransistors (hereinafter referred to as “IGBT's”), metal oxidesemiconductor field effect transistors (hereinafter referred to as“MOSFET's”) and PIN diodes that constitute power semiconductor devices.The invention relates also to the methods for manufacturing these powersemiconductor devices.

BACKGROUND OF THE INVENTION

Generally, power semiconductor devices have been used as non-contactswitches. Therefore, it has been required for the power semiconductordevices to produce lower losses therein. For reducing the lossesproduced therein, techniques for ON-state voltage lowering and switchingloss reduction have been explored. It has been known to persons skilledin the art that there exists a tradeoff relationship between theON-state voltage and the switching (turnoff) loss of power semiconductordevices. The tradeoff relation is called the “ON-state-voltageturnoff-power-loss tradeoff characteristics” for IGBT's and the“forward-voltage reverse-recovery-loss tradeoff characteristics” for PINdiodes.

These tradeoff characteristics are the indices of loss generation in thepower devices that have required improvement. The ON-state-voltageturnoff-power-loss tradeoff characteristics and the soft switchingperformances are not simultaneously improved very often by theconventional methods known to persons skilled in the art. Therefore, ithas been an important problem to improve both the ON-state-voltageturnoff-power-loss tradeoff characteristics and the soft switchingperformances at the same time. Since the turnoff-power-loss reduction isaffected by the high-speed switching performances, it is very importantto improve the high-speed switching performances and the soft switchingperformances at the same time.

The below-listed Patent Document 1 describes a technique for improvingboth the ON-state-voltage turnoff-power-loss tradeoff characteristicsand the soft switching performances by an IGBT. The IGBT includes ann-type silicon layer between first and second major surfaces. Thesilicon layer includes an n-type region and a p-type region. The IGBTalso includes a cathode formed of a first metal film arranged on thefirst major surface and an anode formed of a second metal film coveringthe second major surface. The IGBT is a power semiconductor devicespecifically for high reverse voltage use that includes, as describedfrom the side of the second major surface, a p-type anode region, ann-type field stop layer in contact with the anode region and doped moreheavily than the silicon layer, and the silicon layer in contact withthe field stop layer. The field stop layer is doped with at least onekind of dopant having at least one donor level between the valence bandedge and conduction band edge of silicon and the donor level is far fromthe conduction band edge of silicon by more than 200 meV. PatentDocument 1 describes the use of sulfur and selenium as the dopant.

The below-listed Patent Document 2 proposes a technique for reducing thetotal losses consisting of a turnoff power loss and a steady state lossand for preventing oscillations from occurring on voltage and currentwaveforms. The technique disclosed in the Patent Document 2 forms ann⁺-type buffer region and a first n⁻-type drift region. The thickness ofthe first n⁻-type drift region and the impurity dose amount for formingthe n⁺-type buffer region are determined so that the edge of thedepletion layer expanding in the first n⁻-type drift region, when arated voltage is applied, may stop in the n⁺-type buffer region. Thetechnique disclosed in Patent Document 2 further forms a second n⁻-typedrift region spaced apart from the first n⁻-type drift region by then⁺-type buffer region. The thickness of the second n⁻-type drift regionis set at a predetermined value.

The below listed Patent Document 3 describes a technique for improvingthe forward-voltage reverse-recovery-loss tradeoff characteristics of adiode. The diode includes a first semiconductor layer of a firstconductivity type having a first major surface and a second majorsurface, a second semiconductor layer of the first conductivity typeformed on the first major surface and doped more heavily than the firstsemiconductor layer, and a third semiconductor layer of a secondconductivity type formed on the second major surface. The impurityconcentration and the thickness of the first and third semiconductorlayers are selected so that the electric field caused by a depletionlayer expanding from the pn-junction between the first and thirdsemiconductor layers may be almost in the first semiconductor layer inthe state of sustaining the breakdown voltage and the depletion layermay reach the second semiconductor layer. The cross-sectional area of atleast a portion of the first semiconductor layer parallel to the firstmajor surface thereof is reduced toward the second semiconductor layerfrom the pn-junction between the first and third semiconductor layers.

The below-listed Patent Document 4 describes a technique for improvingthe forward-voltage reverse-recovery-loss tradeoff characteristics andthe soft switching performances of a PIN-diode. The PIN-diode includes afirst n-type drift layer and an n-type buffer layer formed in an n-typedrift layer. The shortest distance from the pn-junction between a p-typeanode layer and the first n-type drift layer to the n-type buffer layerand the width of the n-type buffer layer are set at respectivepredetermined values so that a certain breakdown voltage may be securedand the tradeoff relationship between the high-speed switchingperformance with a low switching loss and the soft recovery performancemay be improved.

The below-listed Patent Document 5 proposes another technique forimproving the tradeoff relationship between the high-speed switchingperformance with a low switching loss and the soft recovery performance.The semiconductor device disclosed in Patent Document 5 includes a firstsemiconductor layer of a first conductivity type; a second semiconductorlayer of a second conductivity type formed on the first major surface ofthe first semiconductor layer, the second semiconductor layer beingdoped more heavily than the first semiconductor layer; and a thirdsemiconductor layer of the first conductivity type formed on the secondmajor surface of the first semiconductor layer, the third semiconductorlayer being doped more heavily than the first semiconductor layer. Thefirst semiconductor layer includes at least a portion in which theimpurity concentration shows the maximum value. The impurityconcentration in the first semiconductor layer reduces gradually towardthe second and third semiconductor layers from the portion in which theimpurity concentration shows the maximum value.

The below-listed Patent Document 6 proposes a technique formanufacturing a semiconductor device, which exhibits a high-speedswitching performance with a low switching loss and a soft switchingperformance. Oxygen is introduced into an n⁻-type FZ wafer thatconstitutes an n⁻-type first semiconductor layer. Then, a p-type secondsemiconductor layer and an anode electrode are formed on the FZ wafer.Protons are irradiated onto the FZ wafer from the anode electrode sideto introduce crystal defects into the FZ wafer. Then, a heat treatmentis performed to make the crystal defects in the FZ wafer recover forsetting the net dopant concentration in a portion of the firstsemiconductor layer to be higher than the initial net dopantconcentration in the FZ wafer and for further forming a desired broadbuffer structure. The manufacturing method proposed in Patent Document 6facilitates manufacturing the semiconductor device, which exhibits thepreferable switching performances described above, from an FZ bulk waferwith low manufacturing costs, with excellent controllability, and withhigh throughput of non-defective products.

The patent documents referenced above are as follows:

-   [Patent Document 1] Published Japanese Translation of PCT    International Publication for Patent Application No. 2002-520885-   [Patent Document 2] Japanese Unexamined Patent Application    Publication No. 2004-193212-   [Patent Document 3] Japanese Patent Publication No. 2573736-   [Patent Document 4] Japanese Unexamined Patent Application    Publication No. 2003-152198-   [Patent Document 5] Japanese Unexamined Patent Application    Publication No. 2003-318412-   [Patent Document 6] International Unexamined Patent Application    Publication No. 2007/055352 Pamphlet

For forming a field stop region and such a region doped more heavilythan the semiconductor substrate in the semiconductor substrate, it isnecessary for the technique described in Patent Document 1 to include athermal diffusion treatment at a relatively high temperature, higherthan 600° C. Especially in manufacturing a device that employs a thinwafer, partings and cracks may be caused in the subsequent metallizationstep. It is difficult to form only a region doped more heavily than thesemiconductor substrate, in the semiconductor substrate. Moreover, theheavily doped region is limited to an n-type region.

According to the technique described in Patent Document 6 formanufacturing a diode, the concentration in the heavily doped regionformed in the semiconductor substrate is constant in parallel to themajor surface of the semiconductor substrate. In order to form a heavilydoped region, the impurity concentration of which changes in parallel tothe major surface of the semiconductor substrate, it is necessary toemploy a method that includes metal mask alignment, for example, with arelatively low accuracy. By the technique described in Patent Document6, the heavily doped region formed in the semiconductor substrate islimited to an n-type heavily doped region. The technique described inPatent Document 3, 5 or 6 makes the voltage rise rate (dV/dt) increaseas the depletion layer reaches the cathode region or the region dopedmore heavily than the semiconductor substrate. Therefore, a softswitching performance is not obtained.

In view of the foregoing, it would be desirable to obviate the problemsdescribed above. It would be also desirable to provide a semiconductordevice that facilitates improving the relevant tradeoff characteristicsand obtaining a soft switching performance. It would be furtherdesirable to provide the method for manufacturing the semiconductordevice that facilitates improving the relevant tradeoff characteristicsand obtaining a soft switching performance. The invention is applicableto an n-type semiconductor substrate as well as to a p-typesemiconductor substrate. The invention is applicable to a semiconductordevice that includes a heavily doped region in the semiconductorsubstrate and to the method for forming such a heavily doped region inthe semiconductor substrate independently of the conductivity type ofthe heavily doped region.

SUMMARY OF THE INVENTION

In a first exemplary aspect of the invention, there is provided a methodof manufacturing a semiconductor device, the method including the stepsof; (a) forming a mask on the first major surface of a semiconductorsubstrate, the mask having openings formed therein; (b) etching theportions of the semiconductor substrate exposed by the openings of themask to form trenches in the first major surface of the semiconductorsubstrate; (c) implanting dopant ions into a semiconductor at thebottoms of the trenches to form impurity layers; (d) activating theimpurity layers into which the dopant ions have been implanted; and (e)filling the trenches with a semiconductor. Depending on the designrequirements of the semiconductor device being manufactured, the step ofactivating, step (d) may be used to cause the impurity layers inadjacent trenches to connect.

In addition to steps (a) through (e) described above, the method ofmanufacturing a semiconductor device according to the present inventionfurther includes the step of (f) flattening the first major surface ofthe semiconductor substrate, step (f) being performed subsequent to step(e).

In addition to steps (a) through (f) described above, the method furtherincludes the step of (g) flattening the second major surface of thesemiconductor substrate, step (g) being performed subsequent to step(f).

With regard to a semiconductor device manufactured according to steps(a) through (f), described above, it is preferable that thesemiconductor substrate flattened through step (g) be 150 μm or less inthickness.

Depending on the design requirements of the semiconductor device beingmanufactured, the method described above may further include: the stepof (h) forming an oxide film on the side walls and the bottom plane ofeach trench, step (h) being performed subsequent to step (b) and priorto step (c); and the step of (k) removing the oxide film, step (k) beingperformed subsequent to step (c) and prior to step (d).

With regard to additional step (h), described above, it is preferablethat the oxide film be 30 nm or more and 100 nm or less in thickness.

Depending on the design requirements of the semiconductor device beingmanufactured, the method may further include the step of (m) removingthe mask, step (m) being performed subsequent to step (d) and prior tostep (e).

With regard to a semiconductor device manufactured according to theabove-described method, the conductivity type of the dopant ispreferably the same as the conductivity type of the semiconductorsubstrate.

Depending on the design requirements of the semiconductor device beingmanufactured, the conductivity type of the semiconductor may be the sameas the conductivity type of the semiconductor substrate.

Depending on the design requirements of the semiconductor device beingmanufactured, the impurity concentration in the semiconductor may bealmost the same as the impurity concentration in the semiconductorsubstrate.

Depending on the design requirements of the semiconductor device beingmanufactured, the impurity concentration in the semiconductor may bedifferent from the impurity concentration in the semiconductorsubstrate.

Depending on the design requirements of the semiconductor device beingmanufactured, the conductivity type of the semiconductor may bedifferent from the conductivity type of the semiconductor substrate.

Depending on the design requirements of the semiconductor device beingmanufactured, the semiconductor may be a single-crystal semiconductorcontaining silicon as a main component thereof.

In a second exemplary aspect of the invention, there is provided asemiconductor device including: a main semiconductor layer of a firstconductivity type, the main semiconductor layer having a first majorsurface and a second major surface, the main semiconductor layer havingan impurity concentration distribution that repeatedly increases anddecreases in a direction parallel to the first major surface thereof; ananode layer of a second conductivity type disposed on the first majorsurface of the main semiconductor layer; an anode electrode disposed onthe anode layer; a cathode layer of the first conductivity type disposedon the second major surface of the main semiconductor layer; a cathodeelectrode disposed on the cathode layer; and a heavily doped layer ofthe first conductivity type formed between the main semiconductor layerand the cathode layer, the heavily doped layer being doped more heavilythan the main semiconductor layer, the heavily doped layer having animpurity concentration distribution that repeatedly increases anddecreases in a direction parallel to the first major surface of the mainsemiconductor layer.

In the semiconductor device according to the second aspect of theinvention, the impurity concentration distribution in the mainsemiconductor layer is produced by lightly doped semiconductor layersand extremely lightly doped semiconductor layers, the lightly dopedsemiconductor layers being doped more heavily than the extremely lightlydoped semiconductor layers, the lightly doped semiconductor layers andthe extremely lightly doped semiconductor layers being shaped incross-section as stripes extending in a direction perpendicular to thefirst major surface of the main semiconductor layer and arrangedalternately and repeatedly in a direction parallel to the first majorsurface of the main semiconductor layer.

In the semiconductor device according to the second exemplary aspect ofthe invention, the junction plane between each of the extremely lightlydoped semiconductor layers and the anode layer is wider than thejunction plane between each of the extremely lightly doped semiconductorlayers and the cathode layer.

In a third exemplary aspect of the invention, there is provided asemiconductor device including: a main semiconductor layer, the mainsemiconductor layer having a first major surface and a second majorsurface, the main semiconductor layer including first semiconductorlayers of a first conductivity type and second semiconductor layers of asecond conductivity type, the first semiconductor layers and the secondsemiconductor layers being arranged alternatively and repeatedly in adirection parallel to the first major surface thereof; a channel regionof the second conductivity type formed in the first major surface of themain semiconductor layer on each of the second semiconductor layers; asource region of the first conductivity type and a gate region of thesecond conductivity type formed in each of the channel regions at thefirst major surface of the main semiconductor layer; a gate insulatorfilm formed on each channel region between the source region and theadjacent first semiconductor layers; a gate electrode disposed on thegate insulator film; a source electrode in contact with the base regionand the source region, with an interlayer insulator interposed betweenthe source electrode and the gate electrode; a substrate layer of thefirst conductivity type disposed on the second major surface of the mainsemiconductor layer; a drain layer of the first conductivity type formedon the substrate layer; a drain electrode disposed on the drain layer;and a heavily doped layer of the first conductivity type between themain semiconductor layer and the substrate layer, the heavily dopedlayer being doped more heavily than the main semiconductor layer, theheavily doped layer having an impurity concentration distribution thatrepeatedly increases and decreases in a direction parallel to the firstmajor surface of the main semiconductor layer.

In a fourth exemplary aspect of the invention, there is provided asemiconductor device including: a main semiconductor layer of a firstconductivity type, the main semiconductor layer having a first majorsurface and a second major surface, the main semiconductor layer havingan impurity concentration distribution that repeatedly increases anddecreases in a direction parallel to the first major surface thereof; achannel layer of a second conductivity type disposed on the first majorsurface of the main semiconductor layer; an emitter region of the firstconductivity type formed selectively in the channel layer; a gateelectrode formed in the channel layer and in proximity to the emitterregion with an insulator film interposed therebetween; an emitterelectrode in contact with the channel layer and the emitter region; acollector layer of the second conductivity type disposed on the secondmajor surface of the main semiconductor layer; a collector electrodedisposed on the collector layer; and a heavily doped layer of the firstconductivity type formed between the main semiconductor layer and thecollector layer, the heavily doped layer being doped more heavily thanthe main semiconductor layer, the heavily doped layer having an impurityconcentration distribution that repeatedly increases and decreases in adirection parallel to the first major surface of the main semiconductorlayer.

In the semiconductor device according to the fourth exemplary aspect ofthe invention, the impurity concentration distribution in the mainsemiconductor layer is produced by lightly doped semiconductor layersand extremely lightly doped semiconductor layers, the lightly dopedsemiconductor layers being doped more heavily than the extremely lightlydoped semiconductor layers, the lightly doped semiconductor layers andthe extremely lightly doped semiconductor layers being shaped incross-section as stripes extending in a direction perpendicular to thefirst major surface of the main semiconductor layer and arrangedalternately and repeatedly in a direction parallel to the first majorsurface of the main semiconductor layer.

In the invention described above, the impurity layers (hereinafterreferred to as the “diffusion layers”) formed in the bottoms of thetrenches by ion implantation and the subsequent heat treatment may beactivated in the activating step, step (d), such that the adjacentdiffusion layers are connected to each other. Therefore, a continuousdiffusion layer is formed at the desired impurity concentration and withthe desired thickness in the depth direction independently of the kindsof the dopant. By changing the depth of the trench, the diffusion layeris formed at the desired location in the semiconductor substrate of thefirst conductivity type. By changing the conductivity type and theimpurity concentration of the semiconductor buried in the trench, asemiconductor substrate that meets the desired design of thesemiconductor device is manufactured. In this manner, a semiconductorsubstrate that provides more degrees of freedom for designing asemiconductor device is manufactured.

The steps, performed after the semiconductor substrate is thinned, aredecreased as compared with the conventional manufacturing method formanufacturing a semiconductor substrate. Therefore, the cracks andpartings caused in the wafer are reduced. By providing the semiconductorsubstrate of the first conductivity type with an impurity concentrationdistribution in parallel to the major surface thereof, a space chargeregion is prevented from expanding at the time of turnoff by the lightlydoped semiconductor layer. The extremely lightly doped semiconductorlayer expands the space charge region to eject the electrons and holesquickly. Therefore, the relevant tradeoff characteristics are improvedand the soft switching performances are obtained at the same time.

By the semiconductor devices and method for manufacturing thesemiconductor devices according to the invention, the relevant tradeoffcharacteristics are improved and the soft switching performances areobtained at the same time. By the manufacturing method according to theinvention, the manufacturing steps for manufacturing a semiconductordevice are reduced and a semiconductor substrate that provides moredegrees of freedom for designing a semiconductor device is manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the structure of asemiconductor device according to a first mode for carrying out theinvention.

FIG. 2 is a flow chart describing the manufacturing process formanufacturing a semiconductor device according to the invention.

FIG. 3 is a cross-sectional view describing an initial step formanufacturing the semiconductor device according to the first mode forcarrying out the invention.

FIG. 4 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 3.

FIG. 5 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 4.

FIG. 6 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 5.

FIG. 7 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 6.

FIG. 8 is a cross-sectional view showing the structure of asemiconductor device according to a second mode for carrying out theinvention.

FIG. 9 is a cross-sectional view describing an initial step formanufacturing the semiconductor device according to the second mode forcarrying out the invention.

FIG. 10 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 9.

FIG. 11 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 10.

FIG. 12 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 11.

FIG. 13 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 12.

FIG. 14 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 13.

FIG. 15 is a cross-sectional view showing the structure of asemiconductor device according to a third mode for carrying out theinvention.

FIG. 16 is a cross-sectional view describing an initial step formanufacturing the semiconductor device according to the third mode forcarrying out the invention.

FIG. 17 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 16.

FIG. 18 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 17.

FIG. 19 is a cross-sectional view showing the structure of asemiconductor device according to a fourth mode for carrying out theinvention.

FIG. 20 is a cross-sectional view describing an initial step formanufacturing the semiconductor device according to the fourth mode forcarrying out the invention.

FIG. 21 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 20.

FIG. 22 is a cross-sectional view showing the structure of asemiconductor device according to a fifth mode for carrying out theinvention.

FIG. 23 is a cross-sectional view describing an initial step formanufacturing the semiconductor device according to the fifth mode forcarrying out the invention.

FIG. 24 is a cross-sectional view describing the manufacturing stepsubsequent to the step described in FIG. 23.

FIG. 25 is a cross-sectional view showing the structure of asemiconductor device according to a sixth mode for carrying out theinvention.

FIG. 26 is a cross-sectional view showing the structure of asemiconductor device according to a seventh mode for carrying out theinvention.

FIG. 27 is a graph showing a set of curves for comparing the turnoffwaveforms of a PIN diode according to a first embodiment and aconventional PIN diode.

FIG. 28 is a graph showing a set of curves for comparing the turnoffwaveforms of a PIN diode according to a second embodiment and theconventional PIN diode.

FIG. 29 is a graph showing a set of curves for comparing the turnoffwaveforms of a PIN diode according to a third embodiment and theconventional PIN diode.

DETAILED DESCRIPTION OF THE PREFERRED MODES FOR CARRYING OUT THEINVENTION

Now the invention will be described in detail hereinafter with referenceto the accompanied drawings, which illustrate the preferred modes forcarrying out the invention.

In the foregoing and following descriptions, electrons are the majoritycarriers in the layer or the region with the prefix “n-type”. In thelayer or the region with the prefix “p-type”, holes are the majoritycarriers. The symbol “+” in superscript after the letter “n” or “p”indicating the conductivity type of the layer or the region, indicatesthat the layer or the region is doped heavily. The symbol “-” insuperscript after the letter “n” or “p” indicating the conductivity typeof the layer or the region, indicates that the layer or the region isdoped lightly.

Throughout the accompanied drawings, the same reference numerals areused to designate the same constituent elements and their duplicateddescriptions are omitted for the sake of simplicity.

First Mode for Carrying Out the Invention

FIG. 1 is a cross-sectional view showing the structure of asemiconductor device according to a first mode for carrying out theinvention.

As shown in FIG. 1, the semiconductor device according to the first modefor carrying out the invention is a trench-gate IGBT including a fieldstop layer. In the surface portion of lightly doped n-type mainsemiconductor layer 1 on the first major surface side thereof, p-typechannel layer (channel region) 2 is formed. In the surface portion ofn-type main semiconductor layer 1 on the second major surface sidethereof, heavily doped p-type collector layer 3 is formed. In n-typemain semiconductor layer 1, n-type base layer 4, in which the impurityconcentration distribution is uniform, is formed between p-type channellayer (channel region) 2 and p-type collector layer 3. Between n-typebase layer 4 and p-type collector layer 3, n⁺-type field stop layer 5 isformed.

In the surface portion of p-type channel layer 2, n⁺-type emitter region6 is formed selectively. In the surface portion of n-type mainsemiconductor layer 1 on the first major surface side thereof,stripe-shaped trenches 7 are formed. Trench 7 is formed adjacent ton⁺-type emitter region 6 and extends from the first major surface ofn-type main semiconductor layer 1 into n-type base layer 4 throughp-type channel layer 2. Gate electrode 9 is disposed in trench 7 withgate insulator film 8 interposed between gate electrode 9 and the innerwall of trench 7.

On the first major surface of n-type main semiconductor layer 1,interlayer insulator film 10 is formed such that interlayer insulatorfilm 10 covers gate electrode 9. On the first major surface of n-typemain semiconductor layer 1, emitter electrode 11 consisting of a metalfilm is formed such that emitter electrode 11 covering interlayerinsulator film 10 is in contact with n⁺-type emitter region 6. In thesurface portion of p-type channel layer 2, n⁺-type body region 12 isformed selectively. Emitter electrode 11 is connected electrically top-type channel layer 2 via p⁺-type body region 12. Collector electrode13 consisting of a metal film is formed on p-type collector layer 3.Although not illustrated in FIG. 1, a passivation film consisting of anitride film, an amorphous silicon film or a polyimide film is formedsometimes on emitter electrode 11.

FIG. 2 is a flow chart describing the manufacturing process formanufacturing the semiconductor device according to the invention.

Referring now to FIG. 2, an n-type semiconductor substrate is prepared(step S1). Then, a mask oxide film having openings therein is formed onthe first major surface of the n-type semiconductor substrate (step S2).Then, trenches are formed in the n-type semiconductor substrate usingthe mask oxide film as a trench-etching mask (step S3). Then, dopantions are implanted into the trench bottoms perpendicular to the firstmajor surface of the n-type semiconductor substrate (step S5). Then, then-type semiconductor substrate is thermally treated to activate theimpurity layer, into which the dopant ions have been implanted (stepS6). Then, the trench is filled with an n-type single-crystalsemiconductor layer, the impurity concentration of which is almost thesame as the impurity concentration in the n-type semiconductor substrate(step S8). Then, the portion of the n-type single-crystal semiconductorlayer protruding from the first major surface of the n-typesemiconductor substrate and the mask oxide film are removed to flattenthe n-type semiconductor substrate (step S9).

According to the first mode for carrying out the invention, the step offorming a screening thermal oxide film on the inner surface (the sidewalls and the bottom plane) of the trench (Step S4) is not performed.The mask oxide film is not removed in the step of removing the maskoxide film (step S7) according to the first mode but the mask oxide filmis removed in the step of flattening the n-type semiconductor substrate(step S9). Therefore, steps S4 and S7 are not performed according to thefirst mode.

FIGS. 3 through 7 are the cross-sectional views of a wafer describingthe manufacturing method according to the first mode for carrying outthe invention.

In the steps 51 and S2 described above, first n-type semiconductorsubstrate 21 is prepared for a starting substrate. After a marker forpositioning a photomask on first n-type semiconductor substrate 21(hereinafter referred to as an “alignment target”) is formed, first maskoxide film 22 is formed on the first major surface of first n-typesemiconductor substrate 21 as shown in FIG. 3. Then, a part of firstmask oxide film 22 is removed by photolithographic and etchingtechniques to expose a part of first n-type semiconductor substrate 21.For example, first mask oxide film 22 is left in a stripe pattern andthe first major surface of first n-type semiconductor substrate 21 isexposed in a stripe pattern.

In the step S3 described above, anisotropic etching such as reactive ionetching (hereinafter referred to as “RIE”) is performed to form deeptrenches 23 in first n-type semiconductor substrate 21 as shown in FIG.4. Then, the inner surface (the side walls and the bottom plane) oftrench 23 is washed with a cleaner such as diluted hydrofluoric acid(hereinafter referred to as “DHF”) and buffered hydrogen fluoride(hereinafter referred to as “BHF”)

Then, in the steps S5 and S6 described above, ions are implanted in thebottom planes of trenches 23 perpendicular to the first major surface offirst n-type semiconductor substrate 21. Then, the implanted atoms arethermally treated to form first diffusion layers 24 in the bottom planesof trenches 23 as shown in FIG. 5. Adjacent first diffusion layers 24are connected to each other. First diffusion layers 24 correspond ton⁺-type field stop layers 5 shown in FIG. 1. Then, a sacrifice oxidefilm is formed on the inner surface of trench 23 and, then, thesacrifice oxide film is removed to remove the damaged layer caused inthe inner surface of trench 23 through the formation thereof togetherwith the sacrifice oxide film.

Then, first n-type semiconductor substrate 21 shown in FIG. 5 istransferred into an epitaxial growth furnace not shown and treatedthermally at around 1000° C. to wash the first major surface of firstn-type semiconductor substrate 21. Subsequently, an etching gas and acarrier gas are fed to the epitaxial growth furnace to clean the innersurfaces of trenches 23. Due to the cleaning, the opening width oftrench 23 is a little bit wider than the bottom width of trench 23.

Then, in the step S8 described above, trenches 23 are filled by theepitaxial growth method with first n-type semiconductor 25 withoutleaving any gap or any void as shown in FIG. 6. The impurityconcentration in first n-type semiconductor 25 is almost the same as theimpurity concentration in first n-type semiconductor substrate 21.

Then, in the step S9 described above, first n-type semiconductor 25grown over first mask oxide film 22 is removed by chemical mechanicalpolishing (hereinafter referred to as “CMP”) and such polishing and thepolished surface is flattened. Then, first mask oxide film 22 isremoved. Semiconductor substrate 101 prepared as described above is usedas a manufacturing substrate for manufacturing a semiconductor device.

Although not illustrated, a surface structure, including p-type channellayer 2, n⁺-type emitter region 6, trench 7, gate insulator film 8, gateelectrode 9, interlayer insulator film 10, emitter electrode 11, andp⁺-type body region 12, is formed by the techniques well known topersons skilled in the art on the first major surface side of amanufacturing substrate (semiconductor substrate 101) for manufacturinga semiconductor device. Polishing and etching are conducted on thesecond major surface of semiconductor substrate 101 to thinsemiconductor substrate 101. Then, p-type collector layer 3 is formed byperforming ion implantation on the second major surface side and bysubsequent heat treatment. The thickness of p-type collector layer 3 is1 μm or less. Then, collector electrode 13 is formed. If necessary, thefirst major surface side is covered with a passivation film. Thus, atrench-gate IGBT as shown in FIG. 1 is completed.

In the descriptions of the trench-gate IGBT according to the first mode,the dimensions, impurity concentrations and process conditions areexemplary and not restrictive. The dimensions, impurity concentrationsand process conditions for a trench-gate IGBT of the 600 V class will bedescribed below.

An n-type floating zone (hereinafter referred to as “FZ”) siliconsubstrate cut out from a silicon ingot manufactured by the FZ method isused for a starting substrate that is first n-type semiconductorsubstrate 21. The resistivity of the FZ silicon substrate is 30 Ωcm. Thethickness of the FZ silicon substrate is 500 μm. The orientation of theplane of the FZ silicon substrate is (100). The direction of orientationflat of the FZ silicon substrate is <100>. First mask oxide film 22 isformed by a dry oxidation treatment conducted at 1100° C. for 9 hr.First mask oxide film 22 is 1.8 μm in thickness. The opening width oftrench 23 is 6 μm and the depth thereof is 70 μm. Trench 23 is not sodeep that trench 23 extends through first n-type semiconductor substrate21. Adjacent trenches 23 are spaced apart for 10 μm.

The conditions in the chamber, in which the step S3 of trench etching isconducted, are described below.

The pressure inside the camber is set at 3 Pa. The gas composition ratioof hydrogen bromide (HBr), sulfur hexafluoride (SF₆) and oxygen (O₂) isset at 1:2:2. The source power is set at 900 W. The bias power is set at100 W.

After the step S5, the damaged layer produced on the inner surface oftrench 23 is removed with a plasma etcher, by chemical dry etching(hereinafter referred to as “CDE”) or by forming a thin sacrifice oxidefilm of 50 nm or less in thickness on the inner surface of trench 23 andremoving the sacrifice oxide film with hydrofluoric acid.

The ion implantation into the bottom of trench 23 is conducted usingphosphorus ions (Pt) for a dopant at a dose amount of 1×10 ¹³ cm⁻².First diffusion layer 24 is treated thermally at 1150° C. for 5 hr. Ahalogen-containing gas such as hydrogen chloride (HCl) is used for anetching gas and hydrogen (H₂) for a carrier gas for cleaning the innersurface of trench 23. It is preferable for the temperature inside thechamber to be 1000° C. or higher. It is preferable for the carrier gaspressure inside the chamber to be around 200 Torr.

In the step S8, the semiconductor gases fed into the chamber includetrichlorosilane (SiHCl₃) for a growth gas, phosphine (PH₃) for a dopinggas, a gas containing hydrogen chloride for an etching gas, and hydrogengas for a carrier gas. The growth gas, doping gas and etching gas areused with respect to the carrier gas at the respective ratios of1:0.1:1.5. It is preferable for the hydrogen gas pressure to be 40 Torror lower. Semiconductor substrate 101 is 70 μm in thickness. The dopantfor forming p-type collector layer 3 is boron (B), for example.Collector electrode 13 is formed by sputtering or by vacuum depositionusing aluminum (Al), titanium (Ti), nickel (Ni), and copper (Cu).

By setting the gas phase diffusion length of hydrogen chloride in thestep of cleaning the inner surface of trench 23 conducted before thestep of filling trench 23, an etching speed difference is caused betweenthe opening and bottom of trench 23. Due to the etching speed differencecaused, the opening of trench 23 is a little bit wider than the bottomof trench 23. By using a chlorine-containing growth gas as one of thegases fed into the chamber in the step of filling trench 23, epitaxialgrowth is conducted while etching is conducted. By the etching, anepitaxial growth speed difference is caused between the opening andbottom of trench 23. Due to the epitaxial growth speed differencecaused, it is possible to bury first n-type semiconductor 25 in thebottom of trench 23 before the opening thereof is corked with firstn-type semiconductor 25. By virtue of the mechanism described above, itis possible to fill trench 23 with first n-type semiconductor 25 withoutleaving any gap or any void.

It is preferable to set n-type main semiconductor layer 1 to be 80 μm orless in thickness, since the ON-state resistance can be reduced bythinning n-type main semiconductor layer 1. It is preferable to set thesacrifice oxide film to be 30 nm or more and 100 nm or less inthickness, since the damaged layer is not removed well when thesacrifice oxide film is 30 nm or less in thickness. When the sacrificeoxide film is 100 nm or more in thickness, it takes longer times to formthe sacrifice oxide film and to remove the sacrifice oxide film,elongating the total process duration.

According to the first mode for carrying out the invention, firstdiffusion layer 24 is formed in the bottom of trench 23 by ionimplantation and the subsequent heat treatment. Therefore, firstdiffusion layer 24 can be formed at a desired impurity concentration andat a desired thickness in the depth direction independently of the kindsof dopant. By changing the depth of trench 23, first diffusion layer 24can be formed at a desired location in first n-type semiconductorsubstrate 21. Therefore, it is possible to manufacture a semiconductorsubstrate that provides more degrees of freedom for designing asemiconductor device. By the manufacturing method according to the firstmode, the number of the manufacturing steps after thinning thesemiconductor substrate is reduced as compared with the conventionalmethod for manufacturing an IGBT. The manufacturing steps reductionfacilitates making less partings and cracks to be caused in the wafer.Since it is not necessary to form a field stop layer and such adiffusion layer after thinning the semiconductor substrate, it is notnecessary to conduct special steps such as bonding a supportingsubstrate to the thinned semiconductor substrate and subsequentlyremoving the supporting substrate.

Second Mode for Carrying Out the Invention

Now the manufacturing method for manufacturing a semiconductor deviceaccording to a second mode for carrying out the invention will bedescribed below. The descriptions of the second mode and with referenceto the attached drawings that duplicate the descriptions of the firstmode will be omitted for the sake of simplicity.

FIG. 8 is a cross-sectional view showing the structure of asemiconductor device according to the second mode. As shown in FIG. 8,the semiconductor device according to the second mode is asuper-junction MOSFET including a buffer layer.

As shown in FIG. 8, the super-junction MOSFET includes epitaxialsubstrate 31 including second n-type semiconductor substrate 51 andn-type epitaxial layer 52 above second n-type semiconductor substrate51. In the surface portion of epitaxial substrate 31 on the second majorsurface side thereof, n-type drain layer 32 is formed on second n-typesemiconductor substrate 51. Above n-type drain layer 32, an alternatingconductivity type structure is formed with second n-type semiconductorsubstrate 51 interposed between the alternating conductivity typestructure and n-type drain layer 32. In the alternating conductivitytype structure, n-type epitaxial layer 52 and p-type semiconductor layer57 in contact with each other are arranged alternately and repeatedly.Between second n-type semiconductor substrate 51 and the alternatingconductivity type structure including n-type epitaxial layers 52 andp-type semiconductor layers 57, n⁺-type buffer layer 35 is formed. Onp-type semiconductor layer 57 in the alternating conductivity typestructure, p-type channel region 36 is formed. In the surface portion ofp-type channel region 36, p-type base region 37 and n-type source region38 are formed.

Gate insulator film 39 is formed along the surface of p-type channelregion 36 between n-type source region 38 and n-type epitaxial layers52. Gate electrode 40 is formed on gate insulator film 39. Sourceelectrode 42 is in contact with p-type base region 37 and n-type sourceregion 38. Source electrode 42 is insulated from gate electrode 40 byinterlayer insulator film 41. Drain electrode 43 is formed on the backsurface of epitaxial substrate 31. The semiconductor device having thestructure described above is covered with a not-shown surface protectorfilm.

The semiconductor substrate according to the second mode is manufacturedin the same manner as the semiconductor substrate according to the firstmode. In the manufacturing process according to the second mode, step S4(of forming a screening oxide film) is performed between step S3 (offorming a trench) and step S5 (of implanting ions into the bottom of thetrench). In step S8 (of filling the trenches), the trenches are filledwith a semiconductor, the conductivity type of which is different fromthe conductivity type of the semiconductor substrate prepared as astarting substrate.

FIGS. 9 through 14 are the cross-sectional views of a wafer describingthe manufacturing method according to the second mode for carrying outthe invention.

In steps S1 through S3, second n-type semiconductor substrate 51 isprepared for a starting substrate and n-type epitaxial layer 52 is grownon second n-type semiconductor substrate 51 according to the second modeas shown in FIG. 9.

Then, an alignment target is formed in the same manner as according tothe first mode. Then, second mask oxide film 53 having openings thereinis formed on n-type epitaxial layer 52. Then, deep trenches 54 areformed in n-type epitaxial layer 52 as shown in FIG. 10. The innersurface (the side walls and the bottom plane) of trench 54 is washed.

Then, in step S4, screening oxide film 55 is formed on the inner surfaceof trench 54 as shown in FIG. 11. Then, in steps S5 and S6, ions areimplanted into second n-type semiconductor substrate 51 in the bottom oftrench 54 through screening oxide film 55 and almost perpendicular tothe first major surface of second n-type semiconductor substrate 51.Then, screening oxide film 55 is removed completely as shown in FIG. 12.By forming screening oxide film 55, unnecessary heavy metals areprevented from mixing into the implanted ions.

Second diffusion layer 56 is made to diffuse in a shape similar to theshape of the diffusion layer according to the first mode (diffusionlayer 24 in FIG. 5). Since the impurity concentration in second n-typesemiconductor substrate 51 is higher than the impurity concentration insecond diffusion layer 56, the portion of second diffusion layer 56diffused inside second n-type semiconductor substrate 51 is not shown inthe drawings. Then, a thermal treatment is performed to form seconddiffusion layers 56 on the boundary planes between second n-typesemiconductor substrate 51 and n-type epitaxial layers 52. Seconddiffusion layers 56 formed as described above correspond to n⁺-typebuffer layers 35 in FIG. 8.

Then, in steps S8 and S9, trenches 54 are filled with second p-typesemiconductor 57 without leaving any gap or any void as shown in FIG. 13and in the same manner as according to the first mode. In fillingtrenches 54, second p-type semiconductor 57, the conductivity type ofwhich is different than that of n-type epitaxial layer 52, is grownepitaxially. Then, the surface of n-type epitaxial layer 52 is flattenedas shown in FIG. 14 and in the same manner as according to the firstmode. Semiconductor substrate 102 manufactured as described above isused for a manufacturing substrate for manufacturing a semiconductordevice.

Although not illustrated, a surface structure is formed by the methodswell known to persons skilled in the art on the first major surface sideof the manufacturing substrate (semiconductor substrate 102) formanufacturing a semiconductor device. The surface structure includesp-type channel region 36, p-type base region 37, n-type source region38, gate insulator film 39, gate electrode 40, interlayer insulator film41, and source electrode 42.

Etching and polishing are performed on the second major surface ofsemiconductor substrate 102 to thin semiconductor substrate 102. Then,ion implantation and the subsequent thermal treatment are performed onthe second major surface side of semiconductor substrate 102 to lowerthe contact resistance of second n-type semiconductor substrate 51. Theportion of second n-type semiconductor substrate 51, the contactresistance of which is lowered, corresponds to n-type drain layer 32.The thickness of n-type drain layer 32 is 1 μm or less. Then, drainelectrode 43 is formed. By the thermal treatment conducted in themanufacturing process for manufacturing the semiconductor device, seconddiffusion layers 56 diffuse into p-type semiconductor layer 57 such thatsecond diffusion layers 56 are on the boundary between second n-typesemiconductor substrate 51 and the alternating conductivity typestructure including n-type epitaxial layer 52 and p-type semiconductorlayer 57 without causing any gap. Second diffusion layer 56 correspondsto n⁺-type buffer layer 35. Thus, the super-junction MOSFET shown inFIG. 8 is completed.

In the descriptions of the super-junction MOSFET according to the secondmode, the dimensions, impurity concentrations and process conditions areexemplary and not restrictive. The dimensions, impurity concentrationsand process conditions for manufacturing a super-junction MOSFET of the600 V class will be described below.

An n-type silicon substrate cut out from a silicon ingot manufactured bythe Czochralski method (hereinafter referred to as the “CZ method”) isused for a starting substrate that is second n-type semiconductorsubstrate 51 according to the second mode. (Hereinafter the n-typesilicon substrate cut out from a silicon ingot manufactured by the CZmethod will be referred to as the “CZ silicon substrate”.) Theresistivity of the CZ silicon substrate is 0.01 Ωcm. The thickness ofthe CZ silicon substrate is 500 μm. The orientation of the plane of theCZ silicon substrate is (100). The direction of orientation flat of theCZ silicon substrate is <100>. The resistivity of n-type epitaxial layer52 is from 5 to 10 Ωcm. The thickness of n-type epitaxial layer 52 is 55μm. Second mask oxide film 53 is 1.8 μm in thickness. Trench 54 is 55 μmin depth. Trench 54 is deep enough to extend through n-type epitaxiallayer 52. Screening oxide film 55 is 100 nm in thickness.

Diborane (B₂H₆) is used for a doping gas fed into the chamber as one ofthe semiconductor material gases. The dose amount of the ions implantedinto the bottom of trench 54 is 1×10 ¹⁴ cm⁻². Semiconductor substrate102 is 250 μm in thickness. In the ion implantation for forming n-typedrain layer 32, arsenic (As) is used as the dopant. Drain electrode 43is formed by depositing titanium (Ti), nickel (Ni), and gold (Au) in theorder of the above description. The other conditions are the same as theconditions according to the first mode.

It is preferable to conduct a treatment for preventing ions from beingimplanted to the side walls of trench 54. It is also preferable toconduct a treatment for preventing heavy metal ions not preferable for adopant from being implanted to the bottom of trench 54. In the samemanner as according to the first mode, it is preferable to conduct thestep of removing the damaged layer caused on the inner surface of thetrench by etching. It is also preferable to conduct the step of washingthe surface of the semiconductor substrate after forming the trench. Itis further preferable to conduct the step of cleaning the inner surfaceof the trench before conducting the step of filling the trench. Byadding these steps, the properties of the semiconductor substrate areimproved and the total number of manufacturing steps is reduced. Achlorine-containing gas is used for a growth gas as effectively asaccording to the first mode.

As described above, the manufacturing method according to the secondmode exhibits the same effects as those exhibited by the manufacturingmethod according to the first mode. Trench 54 is filled with p-typesemiconductor layer 57, the conductivity type of which is different thanthe conductivity type of second n-type semiconductor substrate 51, whichis the starting substrate. By filling trench 54 as described above, asemiconductor substrate including an alternating conductivity type layerformed of n-type epitaxial layer 52 and p-type semiconductor layer 57arranged alternately and repeatedly in parallel to the major surface ofsecond n-type semiconductor substrate 51 is manufactured. In the crosssection shown in FIG. 8, n-type epitaxial layer 52 and p-typesemiconductor layer 57 are shaped with the respective stripes extendingperpendicular to the major surface of second n-type semiconductorsubstrate 51.

Thus, it is possible to manufacture a semiconductor substrate thatprovides more degrees of freedom for designing a semiconductor device.Second diffusion layers 56 formed in the manufacturing substrate formanufacturing the semiconductor device are diffused more and made to bemore uniform by the heat treatment conducted in the process ofmanufacturing the semiconductor device. Therefore, it is not necessaryto add a treatment for further activating second diffusion layers 56 informing n⁺-type buffer layer 35. Even when the depth of trench 54 variesmore or less, n⁺-type buffer layer 35 will be formed with an appropriatethickness between n-type drain layer 32 and the alternating conductivitytype structure formed of n-type epitaxial layers 52 and p-typesemiconductor layers 57.

Third Mode for Carrying Out the Invention

FIG. 15 is a cross-sectional view showing the structure of asemiconductor device according to a third mode for carrying out theinvention.

As shown in FIG. 15, the semiconductor device according to the thirdmode is a PIN diode that includes, in the semiconductor substratethereof, heavily doped regions doped more heavily than the semiconductorsubstrate. As shown in FIG. 15, the PIN diode according to the thirdmode includes p-type anode layer 62 in the surface portion of lightlydoped n-type main semiconductor layer 61 on the first major surface sidethereof. The PIN diode according to the third mode also includes heavilydoped n-type cathode layer 63 in the surface portion of lightly dopedn-type main semiconductor layer 61 on the second major surface sidethereof. Uniformly doped second n-type semiconductor layer 64 is betweenp-type anode layer 62 and n-type cathode layer 63 in n-type mainsemiconductor layer 61. In second n-type semiconductor layer 64, thirdn⁺-type semiconductor layers 65, doped more heavily than second n-typesemiconductor layer 64, are disposed. Anode electrode 66 formed of ametal film is disposed on p-type anode layer 62. Cathode electrode 67formed of a metal film is disposed on n-type cathode layer 63.

According to the third mode, a semiconductor substrate is manufacturedin the same manner as according to the first mode. FIGS. 16 through 18are the cross-sectional views of a wafer describing the manufacturingmethod according to the third mode for carrying out the invention.

In steps S1 through S3, first n-type semiconductor substrate 21 isprepared for a starting substrate and an alignment target is formed onfirst n-type semiconductor substrate 21 according to the third mode.Then, first mask oxide film 22 having openings therein is formed onfirst n-type semiconductor substrate 21 as shown in FIG. 3. Then, deeptrenches 23 are formed in first n-type semiconductor substrate 21 in thesame manner as according to the first mode.

As shown in FIG. 16, protector oxide film 27 is formed on the surface offirst n-type semiconductor substrate 21 and on the inner surfaces oftrenches 23. In steps S5 and S6, ions are implanted almost perpendicularto the first major surface of first n-type semiconductor substrate 21into the bottom of trench 23. Then, mask oxide film 22 and protectoroxide film 27 are removed completely. Then, a heat treatment isconducted to form first diffusion layer 24 in the bottom of trench 23 asshown in FIG. 17. First diffusion layers 24 are formed such that firstdiffusion layers 24 are isolated from each other. First diffusion layer24 corresponds to third n⁺-type semiconductor layer 65 in FIG. 15.

In steps S8 and S9, trench 23 is filled with epitaxially grown firstn-type semiconductor 25 without leaving any gap or any void and, then,the surface of first n-type semiconductor substrate 21 is flattened asshown in FIG. 18. Semiconductor substrate 103 obtained as describedabove is used as a manufacturing substrate for manufacturing asemiconductor device.

Although not illustrated, p-type anode layer 62 is formed by the methodwell known to persons skilled in the art in the surface portion on thefirst major surface side of the manufacturing substrate (semiconductorsubstrate 103) for manufacturing a semiconductor device. Then, anodeelectrode 66 is formed on p-type anode layer 62. Polishing and etchingare conducted on the second major surface of semiconductor substrate 103to thin semiconductor substrate 103. Then, n-type cathode layer 63 isformed by the ion implantation on the second major surface side and bythe subsequent heat treatment. Then, cathode electrode 67 is formed onn-type cathode layer 63. Alternatively, first diffusion layers 24 may beconnected to each other by the heat treatment conducted in the processfor manufacturing the semiconductor device. First diffusion layer 24corresponds to third n⁺-type semiconductor layer 65. Thus, the PIN diodeshown in FIG. 15 is completed.

In the descriptions of the PIN diode according to the third mode, thedimensions, impurity concentrations and process conditions are exemplaryand not restrictive. The dimensions, impurity concentrations and processconditions for manufacturing a diode of the 600 V class will bedescribed below.

According to the third mode, the resistivity of the FZ silicon substrateis 50 Ωcm. The dry oxidation treatment for forming first mask oxide film22 is conducted at 1100° C. for 5 hr. First mask oxide film 22 is 1.2 μmin thickness. Protector oxide film 27 is 50 nm in thickness. Trench 23is 30 μm in depth. Adjacent trenches 54 are spaced apart for 20 μm fromeach other. The dose amount of the ions implanted into the bottom oftrench 23 is 1×10¹² cm⁻². The resistivity of semiconductor substrate 103including trenches 23 filled with first n-type semiconductor 25 is 50Ωcm. Semiconductor substrate 103 is 70 μm in thickness. In the ionimplantation for forming n-type cathode layer 63, phosphorus (P) is usedas the dopant. The metals for forming cathode electrode 67 are the sameas the metals for forming the drain electrode according to the secondmade. The other conditions are the same as the conditions according tothe first mode.

The damaged layer produced during forming trench 23, on the innersurface thereof, can be made to recover by the heat treatment forforming third n⁺-type semiconductor layer 65.

As described above, the manufacturing method according to the third modeexhibits effects the same as those exhibited by the manufacturing methodaccording to the first mode. By forming first diffusion layers 24, asemiconductor substrate exhibiting an impurity concentration variationin parallel to the first major surface of first n-type semiconductorsubstrate 21 is manufactured. Thus, it is possible to manufacture asemiconductor substrate that provides more degrees of freedom fordesigning a semiconductor device.

Fourth Mode for Carrying Out the Invention

FIG. 19 is a cross-sectional view showing the structure of asemiconductor device according to a fourth mode for carrying out theinvention.

As shown in FIG. 19, the semiconductor device according to the fourthmode is a trench-gate IGBT that includes a semiconductor substrateincluding base regions having respective impurity concentrationsdifferent from each other and arranged alternately and repeatedly inparallel with the major surface of the semiconductor substrate. Thetrench-gate IGBT according to the fourth mode has a structure almost thesame as the structure of the trench-gate IGBT according to the firstmode. N-type base layer 4 in the trench-gate IGBT according to the firstmode is replaced by lightly doped n-type base layer 14 and extremelylightly doped n-type base layer 15 in the trench-gate IGBT according tothe fourth mode. Extremely lightly doped n-type base layer 15 is dopedmore lightly than lightly doped n-type base layer 14. Hereinafter,lightly doped n-type base layer 14 will be referred to as “LDN baselayer 14” and extremely lightly doped n-type base layer 15 as “XLDN baselayer 15”. LDN base layer 14 and XLDN base layer 15 in contact with eachother are arranged alternately and repeatedly in parallel with the firstmajor surface of n-type main semiconductor layer 1. In the cross sectionshown in FIG. 19, LDN base layer 14 and XLDN base layer 15 are shapedwith their respective stripes extending perpendicular to the first majorsurface of n-type main semiconductor layer 1.

The manufacturing steps for manufacturing a semiconductor substrateaccording to the fourth mode are substantially the same as themanufacturing steps for manufacturing the semiconductor substrateaccording to the first mode. According to the fourth mode, step S7 (ofremoving the mask oxide film) is conducted between the step S6 (ofactivating the impurity layer) and the step S8 (of filling thetrenches). In the step S8, the trenches are filled with an n-typesemiconductor, the impurity concentration of which is different from theimpurity concentration in the n-type semiconductor substrate preparedfor a starting substrate.

FIGS. 20 and 21 are the cross-sectional views of a wafer describing themanufacturing method for manufacturing the semiconductor deviceaccording to the fourth mode.

In steps S1 through S6, an alignment target is formed on first n-typesemiconductor substrate 21 prepared for a starting substrate in the samemanner as according to the first mode. Then, first mask oxide film 22having openings formed therein is formed on first n-type semiconductorsubstrate 21 as shown in FIG. 3. Then, deep trenches 23 are formed, asshown in FIG. 4, in first n-type semiconductor substrate 21 in the samemanner as according to the first mode. The impurity concentration infirst n-type semiconductor substrate 21 is almost the same with theimpurity concentration in LDN base layer 14. Then, a not-shown protectoroxide film is formed on the surface of first mask oxide film 22 and onthe inner surfaces of trenches 23. Then, ion implantation and thermaldiffusion are conducted to form first diffusion layer 24 in the bottomof trench 23 as shown in FIG. 5 according to the fourth mode. Firstdiffusion layer 24 corresponds to n⁺-type field stop layer 5 in FIG. 19.

Then, in step S7, first mask oxide film 22 is removed completely asshown in FIG. 20. Then, in the steps S8 and S9, trenches 23 are filledwith second n-type semiconductor 28 without leaving any gap not any voidas shown in FIG. 21 in the same manner as according to the first mode.According to the fourth mode, trenches 23 are filled with second n-typesemiconductor 28, the impurity concentration of which is lower than theimpurity concentration in first n-type semiconductor substrate 21. Theimpurity concentration in second n-type semiconductor 28 is almost thesame as the impurity concentration in XDLN base layer 15. Then, secondn-type semiconductor 28 extending outward from the first major surfaceof first n-type semiconductor substrate 21 is removed and the firstmajor surface of first n-type semiconductor substrate 21 is flattened.Semiconductor substrate 104 obtained as described above is used as amanufacturing substrate for manufacturing a trench-gate IGBT. Thesubsequent steps are conducted in the same manner as according to thefirst mode. Thus, a trench-gate IGBT is completed as shown in FIG. 19.

For manufacturing a manufacturing substrate (semiconductor substrate104) from a starting substrate (first n-type semiconductor substrate21), various techniques known to persons skilled in the art areapplicable in the same manner as according to the first mode. Forexample, a thin semiconductor layer is grown epitaxially on the firstmajor surface of a starting substrate. Impurity ions are implanted intosome portions of the epitaxial growth layer such that impurityconcentration variations are caused in the epitaxial growth layer plane.The step of epitaxial growth and the step of ion implantation arerepeated so that a lightly doped semiconductor layer and an extremelylightly doped semiconductor layer may be arranged alternately. Theconductivity type of the lightly doped semiconductor layer is the sameas the conductivity type of the extremely lightly doped semiconductorlayer. But, the impurity concentration in the lightly dopedsemiconductor layer is different than (higher than) the impurityconcentration in the extremely lightly doped semiconductor layer.

In the descriptions of the trench-gate IGBT according to the fourthmode, the dimensions, impurity concentrations and process conditions areexemplary and not restrictive. The dimensions, impurity concentrationsand process conditions for manufacturing a trench-gate IGBT of the 1200V class will be described below.

According to the fourth mode, the resistivity of the FZ siliconsubstrate is 500 Ωcm. The dry oxidation treatment for forming first maskoxide film 22 is conducted at 1100° C. for 18 hr. First mask oxide film22 is 2.4 μm in thickness. The opening width of trench 23 is 10 μm andthe depth thereof is 120 μm. The protector oxide film is 30 nm inthickness. The dose amount of the ions implanted into the bottom oftrench 23 is 5×10¹² cm⁻². The resistivity of semiconductor substrate 104including trenches 23 filled with second n-type semiconductor 28 is 30Ωcm. Semiconductor substrate 104 is 150 μm or less in thickness.Typically, semiconductor substrate 104 is 120 μm in thickness. The otherconditions are the same as the conditions according to the first mode.

According to the fourth mode, the impurity concentration of first n-typesemiconductor substrate 21 used for a starting substrate is set to bethe same as the impurity concentration of LDN base layer 14.Alternatively, the impurity concentration of first n-type semiconductorsubstrate 21 may be set to be the same as the impurity concentration ofXLDN base layer 15 with no problem. And, the impurity concentration ofsecond semiconductor 28 may be set to be the same as the impurityconcentration of LDN base layer 14 with no problem.

As described above, the manufacturing method according to the fourthmode exhibits the same effects as those exhibited by the manufacturingmethod according to the first mode. According to the fourth mode, n-typebase layer 4 is replaced by a semiconductor layer including first n-typesemiconductor substrate 21 and second semiconductor 28 arrangedalternately and repeatedly. Therefore, it is possible to manufacture asemiconductor substrate, in which impurity concentration variations areproduced in a direction parallel to the first major surface of firstn-type semiconductor substrate 21. In n-type base layer 4, the impurityis uniformly distributed perpendicular to the major surface of thesemiconductor substrate. Therefore, the same effect will be obtained,even if the semiconductor substrate is thinned. Thus, it is possible tomanufacture a semiconductor substrate that provides more degrees offreedom for designing a semiconductor device more than the designfreedoms according to the first and third modes.

Fifth Mode for Carrying Out the Invention

FIG. 22 is a cross-sectional view showing the structure of asemiconductor device according to a fifth mode for carrying out theinvention.

As shown in FIG. 22, the semiconductor device according to the fifthmode for carrying out the invention is a PIN diode that includes asemiconductor substrate including a pair of semiconductor layers, theimpurity concentrations of which are different from each other. Thesemiconductor layers are arranged alternately and repeatedly in adirection parallel to the major surface of the semiconductor substrate.In the PIN diode according to the fifth mode, second n-typesemiconductor layer 64 in the PIN diode according to the third mode isformed of lightly doped n-type semiconductor layer (hereinafter referredto as “LDN layer”) 68 and extremely lightly doped n-type semiconductorlayer (hereinafter referred to as “XLDN layer”) 69. XLDN layer 69 isdoped more lightly than LDN layer 68. In the cross section shown in FIG.22, LDN layer 68 and XLDN layer 69 are shaped with their respectivestripes extending perpendicular to the first major surface of n-typemain semiconductor layer 61. LDN layer 68 and XLDN layer 69 are arrangedalternately and repeatedly in parallel to the first major surface ofn-type main semiconductor layer 61. In the PIN diode according to thefifth mode, p-type anode layer 62, n-type cathode layer 63, anodeelectrode 66, and cathode electrode 67 are disposed in the same manneras in the PIN diode according to the third mode.

The manufacturing steps for manufacturing a semiconductor substrateaccording to the fifth mode are substantially the same as themanufacturing steps for manufacturing a semiconductor substrateaccording to the first mode. However, according to the fifth mode, stepS5 (of implanting ions into the bottom of the trench) and step S6 (ofactivating the impurity layer) are not performed. In step S8, thetrenches are filled with an n-type semiconductor, the impurityconcentration of which is different than the impurity concentration inthe n-type semiconductor substrate prepared for the starting substrate,in the same manner as according to the fourth mode.

FIGS. 23 and 24 are the cross-sectional views of a wafer describing themanufacturing method for manufacturing the semiconductor deviceaccording to the fifth mode.

In steps S1 through S3, an alignment target is formed on first n-typesemiconductor substrate 21 prepared as a starting substrate in the samemanner as according to the first mode. Then, first mask oxide film 22having openings therein is formed on first n-type semiconductorsubstrate 21 as shown in FIG. 3. Then, deep trenches 23 are formed infirst n-type semiconductor substrate 21 in the same manner as accordingto the first mode. The inner surface of trench 23 is washed. Then, thefirst major surface of first n-type semiconductor substrate 21 is washedand the inner surface of trench 23 is cleaned in the same manner asaccording to the first mode.

Then, in steps S8 and S9, trenches 23 are filled with second n-typesemiconductor 28 without leaving any gap or any void as shown in FIG. 23and in the same manner as according to the fourth mode. Then, the firstmajor surface of first n-type semiconductor substrate 21 is flattened asshown in FIG. 24, in the same manner as according to the first mode.

Semiconductor substrate 105 obtained as described above is used as amanufacturing substrate for manufacturing a PIN diode. The subsequentsteps are performed in the same manner as according to the third mode.Thus, a PIN diode is completed as shown in FIG. 22.

In the descriptions of the PIN diode according to the fifth mode, thedimensions, impurity concentrations and process conditions are exemplaryand not restrictive.

According to the fifth mode, the resistivity of the FZ silicon substrateis 40 Ωcm. The dry oxidation treatment for forming first mask oxide film22 is performed at 1100° C. for 18 hr. First mask oxide film 22 is 2.4μm in thickness. The opening width of trench 23 is 10 μm and the depththereof is 130 μm. The dose amount of the ions implanted for formingp-type anode layer 62 is 2×10 ¹³ cm⁻². The heat treatment for formingp-type anode layer 62 is performed at 1100° C. for 5 hr. The resistivityof XLDN layer 69 buried in the trenches is 500 Ωcm. Semiconductorsubstrate 105 is 130 μm in thickness. For forming n-type cathode layer63, phosphorus (P) is used as the dopant. The dose amount of phosphorusfor forming n-type cathode layer 63 is 1×10 ¹⁵ cm⁻². The otherconditions are the same as the conditions according to the first mode.

As described above, the manufacturing method according to the fifth modeexhibits effects the same as those exhibited by the manufacturing methodaccording to the fourth mode.

Sixth Mode for Carrying Out the Invention

FIG. 25 is a cross-sectional view showing the structure of asemiconductor device according to a sixth mode for carrying out theinvention.

As shown in FIG. 25, the semiconductor device according to the sixthmode for carrying out the invention is a PIN diode that includes thirdn⁺-type semiconductor layer 65 additionally disposed between n-typecathode layer 63 and an alternating conductivity type layer includingLDN layer 68 and XDLN layer 69 arranged alternately and repeatedly.

The manufacturing steps for manufacturing a semiconductor substrateaccording to the sixth mode are substantially the same as themanufacturing steps for manufacturing a semiconductor substrateaccording to the first mode. In the step S8, the trenches are filledwith an n-type semiconductor, the impurity concentration of which isdifferent than the impurity concentration in the n-type semiconductorsubstrate prepared for a starting substrate in the same manner asaccording to the fifth mode. The semiconductor substrate obtained asdescribed above is used for a manufacturing substrate for manufacturinga PIN diode. The subsequent steps are conducted in the same manner asaccording to the third mode. Finally, a PIN diode is completed as shownin FIG. 25.

In the descriptions of the PIN diode according to the sixth mode, thedimensions, impurity concentrations and process conditions are exemplaryand not restrictive.

According to the sixth mode, phosphorus (P) or selenium (Se) is used asthe dopant for forming third n⁺-type semiconductor layer 65. The doseamount of the ions for forming third n⁺-type semiconductor layer 65 is5×10 12 cm⁻².

The diffusion length for forming third n⁺-type semiconductor layer 65 isaround 10 μm. The other conditions are the same as the conditionsaccording to the fifth mode.

As described above, the manufacturing method according to the sixth modeexhibits effects the same as those exhibited by the manufacturingmethods according to the first and fifth modes.

Seventh Mode for Carrying Out the Invention

FIG. 26 is a cross-sectional view showing the structure of asemiconductor device according to a seventh mode for carrying out theinvention.

In the semiconductor device according to the seventh mode shown in FIG.26, XLDN layer 69 that is wider at its juncture with p-type anode layer62 than at its juncture with n-type cathode layer 63.

The manufacturing steps for manufacturing a semiconductor substrateaccording to the seventh mode are the same as the manufacturing stepsfor manufacturing a semiconductor substrate according to the sixth mode.In step S3, trench 23 is formed such that the opening width of trench 23is wider than the bottom width thereof. The subsequent steps areconducted in the same manner as according to the third mode. Finally, aPIN diode is completed as shown in FIG. 26.

In the descriptions of the PIN diode according to the seventh mode, thedimensions, impurity concentrations and process conditions are exemplaryand not restrictive.

According to the seventh mode, the resistivity of the FZ siliconsubstrate is 40 Ωcm, and the resistivity of the lightly doped n-typesemiconductor layer 68 is 40 Ωcm for example. The resistivity of XLDNlayer 69 buried in the trench is 500 Ωcm. The other conditions are thesame as the conditions according to the sixth mode.

As described above, the manufacturing method according to the seventhmode exhibits effects the same as those exhibited by the manufacturingmethods according to the first and fifth modes.

Embodiments

A PIN diode is manufactured by the manufacturing method according to thefifth mode for carrying out the invention. An n-type floating-zonesilicon substrate cut out from a silicon ingot prepared by the floatingzone method (hereinafter referred to as the “FZ method”) is used as astarting substrate. (Hereinafter the floating-zone silicon substratewill be referred to as the “FZ silicon substrate”.) The resistivity ofthe FZ silicon substrate is 40 Ωcm. The thickness of the FZ siliconsubstrate is 500 μm. The orientation of the plane of the FZ siliconsubstrate is (100). The direction of orientation flat of the FZ siliconsubstrate is <100>.

A mask oxide film of 2.4 μm in thickness is formed, for example, by athermal oxidation treatment on the first major surface of the FZ siliconsubstrate. Then, the mask oxide film is removed by photolithographic andetching techniques for a width of 10 μm and with spaces of 10 μm toexpose the first major surface of the FZ silicon substrate in a stripepattern. Then, the silicon layer is removed in rectangular shapes to thedepth of around 130 μm by anisotropic etching such as RIE using theremaining mask oxide film for a mask to form stripe-shaped deep trenchesin the FZ silicon substrate. The portions of the FZ silicon substratebetween the trenches provide LDN layers.

Then, epitaxial growth is performed to fill the trenches in the FZsilicon substrate with an n-type semiconductor having a resistivity of500 Ωcm. The epitaxial growth layers comprise the XLDN layers of thedevice. Then, polishing such as CMP is performed to remove the epitaxialgrowth layer grown over the mask oxide film and the polished plane isflattened. Thus, a manufacturing substrate for manufacturing asemiconductor device is obtained. A PIN diode having the structure shownin FIG. 22 is manufactured using the manufacturing substrate obtained asdescribed above. After forming the anode side structure (front surfacestructure) of the diode, the substrate is polished and etched from theback surface side thereof to thin the substrate to around 130 μm inthickness. This substrate thickness is the thickness of the n-type mainsemiconductor layer. Then, the cathode side structure (back surfacestructure) of the diode is formed. The PIN diode manufactured asdescribed above and having the structure shown in FIG. 22 will bedesignated as the “first diode”.

A PIN diode according to the sixth mode is manufactured utilizing thesteps by means of which the first diode is manufactured, and additionalsteps performed after the stripe-shaped deep trenches are formed. Theadditional steps of implanting ions and thermally treating the implantedatoms are performed to form a diffusion layer in the bottom of thetrenches. The PIN diode manufactured as described above and having thestructure shown in FIG. 25 will be designated as the “second diode”.

A PIN diode according to the seventh mode is manufactured in the samemanner as the second diode except that the opening of the trench is madewider than the bottom thereof. The PIN diode manufactured as describedabove and having the cross-sectional structure shown in FIG. 26 will bedesignated as the “third diode”. In the third diode, the resistivity ofthe FZ silicon substrate is 40 Ωcm and the trench in the FZ siliconsubstrate is filled with a semiconductor, the resistivity of which is500 Ωcm.

A fourth PIN diode is manufactured as a comparative example. Thecomparative PIN diode is a diode according to the first embodiment butthe impurity concentration distribution in the FZ silicon substratethereof is uniform. The comparative diode manufactured as describedabove will be designated as the “conventional diode”.

FIG. 27 is a graph showing a set of curves comparing the turnoffwaveforms of the first diode and the conventional diode. FIG. 28 is agraph showing a set of curves comparing the turnoff waveforms of thesecond diode and the conventional diode. FIG. 29 is a graph showing aset of curves comparing the turnoff waveforms of the third diode and theconventional diode.

As these figures indicate, the reduction rate of the reverse recoverycurrents of the first through third diodes after the reverse recoverycurrents reach their peaks is smaller than the reduction rate of thereverse recovery current of the conventional diode after the reverserecovery current reaches its peak. The currents after they decrease, theso-called tail currents, of the first through third diodes reach zeromore quickly than the tail current of the conventional diode. Theinitial reverse recovery voltage A and ΔV_(PN) max of the first throughthird diodes caused by the reduction rate of the reverse recoverycurrent and the wiring inductance become small. The first through thirddiodes exhibit a turnoff loss almost the same as the turnoff loss thatthe conventional diode exhibits. However, the first through third diodessuppress the cathode voltage rise and quicken the overall turnoff. Inother words, the first through third diodes realize high-speed turnoffand soft switching at the same time.

The first and second diodes exhibit almost the same effects. The firstand third diodes exhibit almost the same effects.

As the results described above indicate, the XLDN semiconductor layermakes the excess carrier ejection start earlier than in the conventionalPIN diode under a low voltage applied in the early stage of a turnoff.The reverse recovery current decreases more slowly than in theconventional PIN diode and, therefore, the oscillation phenomenaexperienced during the reverse recovery of a diode are prevented fromoccurring. By preventing the oscillation phenomena from occurring,electromagnetic noises are prevented.

As described herein, the invention is particularly effective forvertical power semiconductor devices such as IGBT's, MOSFET's and PINdiodes and for manufacturing such devices. However, although theinvention has been described in connection with the modes for carryingout the invention, various changes, modifications and adaptations willbe apparent to persons skilled in the art without departing from thetrue spirit of the invention. Such changes, modifications andadaptations are intended to be comprehended within the meaning and rangeof equivalents of the appended claims.

For example, it is not always necessary to shape the lightly dopedsemiconductor layers and the extremely lightly doped semiconductorlayers, which are arranged alternately and repeatedly in the mainsemiconductor layer, as stripes. Alternatively, the lightly dopedsemiconductor layers or the extremely lightly doped semiconductor layersmay be shaped as islands distributed in the extremely lightly dopedsemiconductor layer or in the lightly doped semiconductor layer, as thecase may be, with no problem. In the main semiconductor layer, theimpurity concentration distribution may have three or more peaks and theimpurity concentration peaks may be repeated with no problem.

Still alternatively, the buffer layer may be a broad buffer layer withno problem. The broad buffer layer may be, for example, an n⁻-type driftlayer in a MOSFET. The average concentration in a direction parallel tothe first major surface in the n⁻-type drift layer has a peak (shows amaximum) in the vicinity of the middle portion of the n⁻-type driftlayer and reduces gradually toward the cathode and anode of the MOSFET.In the trench-gate IGBT, it is not always necessary for the pitch, atwhich a pair of the lightly doped semiconductor layer and the extremelylightly doped semiconductor layer is repeated, to be constant. Also inthe super-junction MOSFET, it is not always necessary for the pitch, atwhich a pair of the n-type semiconductor layer and the p-typesemiconductor layer is repeated, to be constant.

Moreover, the invention is applicable not only to the PIN diodes butalso to merged pin and Schottky (MPS) diodes, bipolar transistors andother devices.

In the descriptions of the modes for carrying out the invention, thedimensions, impurity concentrations and process conditions are exemplaryand not restrictive. Although the first conductivity type is an n-typeand the second conductivity type is a p-type in the above descriptions,the first conductivity type may be a p-type and the second conductivitytype may be an n-type with no problem.

It will be apparent to persons skilled in the art that the manner ofpracticing the claimed invention has been adequately disclosed in theabove description of the preferred modes and embodiments taken togetherwith the drawings.

1. A semiconductor device comprising: a main semiconductor layer of afirst conductivity type, the main semiconductor layer having a firstmajor surface and a second major surface opposite the first majorsurface, the main semiconductor layer having an impurity concentrationdistribution that repeatedly increases and decreases in a directionparallel to the first major surface thereof; an anode layer of a secondconductivity type disposed on the first major surface of the mainsemiconductor layer; an anode electrode disposed on the anode layer; acathode layer of the first conductivity type disposed on the secondmajor surface of the main semiconductor layer; a cathode electrodedisposed on the cathode layer; and a heavily doped layer of the firstconductivity type formed between the main semiconductor layer and thecathode layer, the heavily doped layer being doped more heavily than themain semiconductor layer, the heavily doped layer having an impurityconcentration distribution that repeatedly increases and decreases in adirection parallel to the first major surface of the main semiconductorlayer.
 2. The semiconductor device according to claim 1, wherein theimpurity concentration distribution in the main semiconductor layercomprises lightly doped semiconductor layers and extremely lightly dopedsemiconductor layers, the lightly doped semiconductor layers being dopedmore heavily than the extremely lightly doped semiconductor layers, thelightly doped semiconductor layers and the extremely lightly dopedsemiconductor layers being shaped in cross-section as stripes extendingin a direction perpendicular to the first major surface of the mainsemiconductor layer and arranged alternately and repeatedly in adirection parallel to the first major surface of the main semiconductorlayer.
 3. The semiconductor device according to claim 2, wherein ajunction plane between each of the extremely lightly doped semiconductorlayers and the anode layer is wider than a junction plane between eachof the extremely lightly doped semiconductor layers and the cathodelayer.
 4. A semiconductor device comprising: a main semiconductor layerhaving a first major surface and a second major surface opposite thefirst major surface, the main semiconductor layer comprising firstsemiconductor layers of a first conductivity type and secondsemiconductor layers of a second conductivity type, the firstsemiconductor layers and the second semiconductor layers being arrangedalternately and repeatedly in a direction parallel to the first majorsurface thereof; a channel region of the second conductivity type formedin the first major surface of the main semiconductor layer on each ofthe second semiconductor layers; a source region of the firstconductivity type and a gate region of the second conductivity typeformed in each of the channel regions, proximate to the first majorsurface of the main semiconductor layer; a gate insulator film formed oneach channel region between the source region and the adjacent firstsemiconductor layers; a gate electrode disposed on each gate insulatorfilm; a source electrode in contact with the base region and the sourceregion, with an interlayer insulator film interposed between the sourceelectrode and each gate electrode; a substrate layer of the firstconductivity type disposed on the second major surface of the mainsemiconductor layer; a drain layer of the first conductivity type formedon the substrate layer; a drain electrode disposed on the drain layer;and a heavily doped layer of the first conductivity type formed betweenthe main semiconductor layer and the substrate layer, the heavily dopedlayer being doped more heavily than the main semiconductor layer, theheavily doped layer having an impurity concentration distribution thatrepeatedly increases and decreases in a direction parallel to the firstmajor surface of the main semiconductor layer.
 5. A semiconductor devicecomprising: a main semiconductor layer of a first conductivity type, themain semiconductor layer having a first major surface and a second majorsurface opposite the first major surface, the main semiconductor layerhaving an impurity concentration distribution that repeatedly increasesand decreases in a direction parallel to the first major surfacethereof; a channel layer of a second conductivity type disposed on thefirst major surface of the main semiconductor layer; an emitter regionof the first conductivity type formed selectively in the channel layer;a gate electrode formed in the channel layer in proximity to the emitterregion with an insulator film interposed therebetween; an emitterelectrode in contact with the channel layer and the emitter region; acollector layer of the second conductivity type disposed on the secondmajor surface of the main semiconductor layer; a collector electrodedisposed on the collector layer; and a heavily doped layer of the firstconductivity type formed between the main semiconductor layer and thecollector layer, the heavily doped layer being doped more heavily thanthe main semiconductor layer, the heavily doped layer having an impurityconcentration distribution that repeatedly increases and decreases in adirection parallel to the first major surface of the main semiconductorlayer.
 6. The semiconductor device according to claim 5, wherein theimpurity concentration distribution in the main semiconductor layercomprises lightly doped semiconductor layers and extremely lightly dopedsemiconductor layers, the lightly doped semiconductor layers being dopedmore heavily than the extremely lightly doped semiconductor layers, thelightly doped semiconductor layers and the extremely lightly dopedsemiconductor layers being shaped in cross-section as stripes extendingin a direction perpendicular to the first major surface of the mainsemiconductor layer and arranged alternately and repeatedly in adirection parallel to the first major surface of the main semiconductorlayer.